Determination of total phosphate in waste waters by on-line microwave digestion incorporating colorimetric detection

Abstract

A flow injection (Fl) system for on-line microwave digestion of waste water samples with determination of total phosphate by means of colorimetric detection is described. Acidified samples are introduced into a water carrier stream and digested under continuous-flow conditions in thin-bore tubing. Following digestion, the samples are cooled on-line and then combined with further reagent streams for subsequent colorimetric detection based on the reduction of heteromolybdophosphoric acid to Molybdenum Blue as an Fl peak at 690 nm. Signals from the flow-through detector are recorded as peak height on a chart recorder. Optimization of parameters such as digestion tube length, digestion tube diameter and reagent concentrations is discussed. Calibration was found to be linear up to 20 ppm of phosphate with a limit of detection of 0.10 ppm. Samples of waste water can be analysed at a rate of approximately 2 min per sample with typical sample relative standard deviations of <5% being achieved. Results for a range of samples were found to agree with those obtained by a conventional ‘block’ digestion autoanalyser method. The pre-treatment of samples with pyrophosphate phosphohydrolase ensured that the determination of total phosphate as orthophosphate could be achieved for samples containing pyrophosphate species.